Many of the technological advances of the last century are the result of researchers unlocking different types of electromagnetic radiation, the waves—like x-rays, microwaves and infrared radiation—that carry energy throughout the universe. There’s one bit of the electromagnetic spectrum, however, called the terahertz region, that researchers have struggled to access. Mastering terahertz waves has huge potential for applications in communications, medical imaging, chemical sensing and many other fields.
That’s why researchers working in the lab of Jun Xiao, an assistant professor of materials science and engineering at the University of Wisconsin-Madison, are studying a new generation of ultrathin 2D quantum materials that can efficiently generate and detect terahertz waves, allowing them to one day be incorporated into electronic devices.
In a paper published on June 12, 2025, in the journal Nature Electronics, Xiao’s group reports the development of a material called a 2D correlated topological semimetal that excels at detecting terahertz signals. The work is a collaboration with materials science and engineering colleague Daniel Rhodes, electrical and computer engineering faculty members Ying Wang and Daniel Van Der Weide and Yang Zhang at the University of Tennessee.
This is in addition to another material, a 2D ferroelectric semiconductor, that Xiao’s group found generates terahertz signals an order of magnitude more efficiently than a widely-used benchmark material. That work, conducted in collaboration with materials science and engineering colleague Yuan Ping and Bing Lv at the University of Texas at Dallas, appeared in the February 2025 issue of Advanced Optical Materials and was featured on the cover.
Together, these studies are moving high-performance terahertz technology based on 2D quantum materials toward reality.
“Terahertz technology is critical in quantum information technology and biomedical sensing, because its frequency resonates with low-energy information carriers, like phonons and magnons, found in quantum materials and molecular vibrations in biological matter,” says Xiao. “Moreover, the ultrahigh bandwidth of the terahertz band could enable new types of high-speed wireless communication.”
Postdoctoral scholar Sujan Subedi (left), Assistant Professor Jun Xiao and PhD student Tairan Xi are working on materials that utilize the terahertz band, which could lead to new biosensors, faster communications and other applications. Photos: Joel Hallberg
Researchers have discovered several materials that can produce and detect terahertz waves, including several organic and inorganic materials that produce the waves when blasted with laser energy. However, all of these materials have drawbacks, like instability at room temperature and incompatibility with the substrates used in semiconductor manufacturing, meaning they can’t be easily integrated into current electronics.
One 2D ferroelectric semiconducting material called niobium oxide diiodide, however, appears to overcome those problems. Not only is it stable at room temperature, but it is also compatible with a large range of substrates due to its weak van der Waals interlayer bonding.
To test the material’s potential as a terahertz emitter, the Xiao group developed a comprehensive ultrafast characterization system, finding that niobium oxide diiodide produced terahertz emissions with an order of magnitude better efficiency than zinc telluride, a nonlinear crystal and the current benchmark material. By employing what’s called a double-pump scheme, the team also found that they could coherently control the amplitude and phase of the terahertz emissions at the ultrafast time scale.
The research confirmed that the material is a possible candidate for use in terahertz devices, but also illuminated principles behind the way the material and light interact, which will help in finding and utilizing other terahertz generators.
“We were actually surprised by the material,” says Sujan Subedi, the first author of the Advanced Optical Materials paper and a postdoc in Xiao’s group. “We supposed it would be very strong, but its large ferroelectricity and in-phase stacking orders amazingly result in an order of magnitude stronger efficiency than zinc telluride.”
Generating terahertz emissions is only one half of the equation; researchers also need to find materials that can detect and process terahertz signals. Current terahertz detectors based on thermal processes are bulky and slow, while those based on electronics like Schottky diodes and high electron mobility transistors have a narrow detection bandwidth and low responsivity.
For the study featured in Nature Electronics, the team looked in another direction, focusing on a class of quantum materials called 2D topological semimetals, with properties that can lead to strong nonlinear light-matter interactions and new terahertz detection mechanisms. In particular, they synthesized, characterized and modeled a 2D material called tantalum iridium telluride.
“The property of this topological semimetal enables a special phenomenon called the nonlinear Hall effect, which can rectify or double the frequency of an AC electrical input.” says Ying Wang, a co-author of the study. “This was first predicted in 2015 and has been studied only up to a few kilohertz. Down to the atomically thin limit, this effect can be further boosted by strong electron correlation effect.”
Tairan Xi, first author of the paper and a PhD student in Xiao’s group, says they pushed the semimetal to new limits. “Other researchers never used it in the terahertz regime, which is much higher,” he says. “We found this new mechanism can indeed be used for efficient terahertz detection with high responsivity, ultrafast and broadband response.”
While it’s possible these specific materials may eventually make it into terahertz electronics, the exploration of the fundamental physics behind them will guide the search for even better materials.
“The exciting results reported by these two works highlight the great potential of 2D materials for next-generation terahertz devices,” says Xiao. “Inspired by this, we are continuing to explore their structure–property relationships and light–matter interactions, aiming to enable ultrafast electronics and optoelectronics applicable to both classical and quantum regimes.”
Other UW-Madison authors of the Advanced Optical Materials paper include Wuzhang Fang, Carter Fox and Fan Fei. Other authors include Wenhao Liu and Zixin Zhai of the University of Texas at Dallas.
The authors acknowledge support from the National Science Foundation-funded University of Wisconsin Materials Research Science and Engineering Center (DMR-2309000), the NSF (award DMR-2237761; and the Office of Naval Research (award N00014-24-1-2068).
Other UW-Madison authors of the Nature Electronics paper include Haotian Jiang, Yangchen He, Yuchen Gu, Carter Fox, Yulu Mao, and Jack Rollins. Other authors include Jiangxu Li, Louis Primeau and Yang Zhang of the University of Tennessee and Takashi Taniguchi and Kenji Watanabe of the National Institute for Materials Science in Tsukuba, Japan.
The authors acknowledge support from the Office of Naval Research (award N00014-24-1-2068); the NSF (award DMR-2237761); the Department of Energy Office of Basic Energy Sciences (award DE-SC0024176); and the NSF-supported University of Wisconsin Materials Research Science and Engineering Center (award DMR-2309000).